Breast cancer is one of the most problematic types of human cancer. The etiology of breast cancer involves a complex interplay of genetic, hormonal, and dietary factors. Attempts to gain a clear understanding of how these factors participate in mammary tumorigenesis have been hampered, in part, by a lack of information on the specific genetic lesions that contribute to the initiation and/or evolution of tumor development. Our recent studies have resulted in identifying and cloning int-5 genomic DNA, a unique and highly conserved novel mouse mammary tumor virus (MMTV) integration locus gene from the BALB/c precancerous D2 hyperplastic alveolar nodule(HAN). Int-5 is different from other int genes: it has been found in precancerous, rather than cancerous neoplasms; it has been found in chemically-, rather than virus-induced neoplasms; and it is expressed in a variety of normal tissues, including the mammary gland. Int-5 is also different from other oncogenes implicated in breast cancer (c-myc, c-Ha-ras, c-erbB and HER-2/neu) because of its possible role in early preneoplastic changes. Overexpression of int-5 in mammary and breast tumors and also in breast cancer cell lines suggests its involvement in mammary carcinogenesis. Therefore, int-5 gene is a good candidate for investigating the role of this gene in mammary cancer. Our objective is to test the transforming ability of int-5 gene directly to establish its role in breast cancer using in vitro cell culture and in vivo transgenic animal models. Our specific aims are: We will 1) clone mouse and human int-5 cDNAs using mouse genomic probe and characterize these clones by determining their nucleotide sequence; 2) determine the nucleotide sequence and structural organization of int-5 genomic clone; 3) establish the transforming ability of int-5 gene using MMTV-int-5 expression vector in cell culture and transgenic animal model systems; transform normal human breast epithelial cells (MCF-10) with MMTV-int-5 fusion gene and examine whether overexpression of this gene in transformed cells leads to tumor formation in nude mice. Also, we will generate int-5 transgenic animals, examine these animals during different stages of mammary gland development and pregnancy/lactation for formation of HAN and their progression to mammary tumors, as well as examine any increased incidence of mammary cancer in these animals; 4) establish the prognostic significance of int-5 overexpression by analyzing primary breast tumors for int-5 overexpression and comparing these data with other clinical factors. These studies will provide basic knowledge and help in understanding the characteristics of int-5 gene and its encoded protein, as well as the transforming ability of int-5 gene and its role in breast cancer. Transgenic animals also provide a model to study the incidence of breast cancer, to determine how this incidence can be reduced by various interventions, and to study the synergistic effect of other oncogenes, if any, in the activation of this gene in mammary neoplasms.